The present invention relates to a semiconductor laser device to be used for optical disks, and the like, and a method of producing the same. Particularly, the present invention relates to a semiconductor laser device having a window structure, which is superior in high power operation characteristics, and a method of producing the same.
In recent years, various types of semiconductor lasers have widely been used as light sources for optical disc devices. In particular, high-power semiconductor lasers have been used as light sources for writing information to disks in a DVD (digital versatile disc) player and a CD-RAM (random access memory) drive, and there is a need for reduction of driving current and further improvement in power.
One of factors that increase the driving current of a semiconductor laser is diffusion of impurity atoms from a cladding layer into an active layer. Further, one of factors that restrict increase in power of a semiconductor laser is catastrophic optical damage (COD), which tends to occur with increase of the optical power density in regions of an active layer in proximity of end faces of a laser cavity.
One method for reducing the driving current in a semiconductor laser device by suppressing the diffusion of impurity atoms into an active layer is adopted in a semiconductor light emitting device disclosed in Patent Document 1 (JP 11-87831 A). Further, as one method for increasing output power by reducing the COD level, there is a method utilizing a window structure in which an active layer of a multiquantum well structure is disordered, as adopted in a semiconductor laser disclosed in Patent Document 2 (JP 3-208388 A).
First, prior art for suppressing diffusion of impurity atoms into an active layer disclosed in Patent Document 1 will be described. FIG. 14 is a cross-sectional view showing an AlGaInP semiconductor laser device as a semiconductor light-emitting device disclosed in Patent Document 1.
In FIG. 14, reference numeral 1 denotes an n-type GaAs substrate, reference numeral 2 denotes an n-type Alx1Gay1Inz1P (0≦x1, y1, z1≦1) cladding layer, reference numeral 3 denotes an Alx2Gay2Inz2P (0≦x2, y2, z2≦1) optical waveguide layer (light guide layer), reference numeral 4 denotes a MQW structure active layer composed of Gay3Inz3P (0≦y3, z3≦1) quantum well layers (well layers) and Alx2Gay2Inz2P barrier layers, reference numeral 5 denotes an Alx2Gay2Inz2P optical waveguide layer (light guide layer), reference numeral 6 denotes a p-type Alx1Gay1Inz1P cladding layer, reference numeral 7 denotes a p-type GaInP intermediate layer, and reference numeral 8 denotes a GaAs cap layer. Reference numeral 10 denotes a p-side electrode, and reference numeral 11 denotes an n-side electrode.
An upper part of the p-type cladding layer 6, the p-type intermediate layer 7 and the p-type cap layer has a stripe shape extending in one direction, and both sides of the stripe portion are filled with an n-type GaAs current constriction layer (current block layer) 9. Se atoms are used as n-type conductivity impurities to be introduced into the n-type cladding layer 2 and the n-type current constriction layer 9, and Zn atoms are used as p-type conductivity impurities to be introduced into the p-type cladding layer 6, the p-type intermediate layer 7 and the p-type cap layer 8.
In the AlGaInP semiconductor laser device with the above construction, the n-type cladding layer 2 has a lattice mismatch rate of −0.15% or more but not more than −0.02% relative to the n-type substrate 1. The p-type cladding layer 6 has a lattice mismatch rate of 0.02% or more but not more than 0.3% relative to the n-type substrate 1.
Next, prior art for reducing the COD level disclosed in Patent Document 2 will be described. FIG. 15A and FIG. 15B are cross-sectional views showing the structure of a semiconductor laser device with a window structure disclosed in Patent Document 2.
FIG. 15A is a cross-sectional view of the semiconductor laser device in an excitation region (active region), and FIG. 15B is a cross-sectional view of the semiconductor laser device in an impurity diffusion region (window region). Reference numeral 21 denotes an n-type GaAs substrate, reference 22 denotes an n-type GaAs buffer layer, reference numeral 23 denotes an n-type AlGaInP cladding layer, reference numeral 24 denotes an undoped GaInP active layer, reference numeral 25 denotes a p-type AlGaInP inner cladding layer, reference numeral 27 denotes a p-type AlGaInP outer cladding layer, reference numeral 29 denotes a p-type GaAs cap layer, reference numeral 30 denotes a GaAs block layer, reference numeral 31 denotes a p-type GaAs contact layer, reference numeral 32 denotes a p-side electrode, and reference numeral 33 denotes an n-side electrode.
FIG. 16A through FIG. 16D are process drawings showing a conventional method of producing a semiconductor laser device disclosed in Patent Document 2. In accordance with FIG. 16A through FIG. 16D, the conventional method of producing a semiconductor laser device will be described.
As shown in FIG. 16A, an n-type GaAs buffer layer 22, an n-type AlGaInP cladding layer 23, an undoped GaInP active layer 24, a p-type AlGaInP inner cladding layer 25, a p-type GaInP etching stopper layer 26, a p-type AlGaInP outer cladding layer 27, a p-type GaInP hetero-barrier layer 28, and a p-type GaAs cap layer 29 are formed in sequence on an n-type GaAs substrate 21 at a growth temperature of 660° C. by an MOVPE (metal organic vapor phase epitaxy) method. Zn atoms are doped, as p-type impurities, into each of the layers having p-type conductivity from the p-type inner cladding layer 25 to the p-type cap layer 29.
Next, a dielectric film 34 is deposited on the p-type cap layer 29, and, after patterning the dielectric film in a stripe shape by photolithography, Zn atoms are diffused by a sealed tube diffusion method using ZnAs2 as an impurity diffusion source. Thereby, highly concentrated Zn atoms are diffused into a region of the undoped active layer 24, which becomes an impurity diffusion region, so that the bandgap energy of the undoped active layer 24 increases.
Next, as shown in FIG. 16B, using the photolithography again, a resist stripe mask 35 is formed on the dielectric film 34 and the p-type cap layer 29. Thereafter, the dielectric film 34, the p-type cap layer 29, the p-type hetero-barrier layer 28 and the p-type outer cladding layer 27 are sequentially removed by a chemical etching treatment as shown in FIG. 16C, so as to form a ridge.
Next, as shown in FIG. 16D, after removing the resist stripe mask 35, using the dielectric film 34 as a mask, an n-type GaAs block layer 30 (see FIG. 15A and FIG. 15B) is selectively grown at a growth temperature of 660° C. by the MOVPE method. Thereby, the n-type block layer 30 is formed in regions on both sides of the ridge, and also on the impurity diffusion regions. Current injection into the regions where the n-type block layer 30 is formed is prevented.
Next, after removing the dielectric film 34, a p-type GaAs contact layer 31 is formed at a growth temperature of 660° C. by the MOVPE method (see FIGS. 15A and 15B). Thereafter, as shown in FIG. 15A and FIG. 15B, a p-side electrode 32 is formed on the p-type contact layer 31, and an n-side electrode 33 is formed on the underside of the n-type substrate 21. Then, the wafer is cleaved, and a semiconductor laser device shown in FIG. 15A and FIG. 15B is obtained.
However, the conventional semiconductor laser devices have the following problems. Specifically, in the semiconductor laser device disclosed in Patent Document 1 in which diffusion of impurity atoms into the active layer is suppressed, in order to suppress diffusion of Zn atoms contained in the p-type cladding layer 6 into the active layer 4, a strain is provided to the p-type cladding layer 6 so that the p-type cladding layer 6 has a lattice mismatch rate of 0.02% or more but not more than 0.3% relative to the n-type substrate 1.
However, in the conventional semiconductor laser device in which the diffusion of impurity atoms into the active layer is suppressed, mere provision of a strain in the p-type cladding layer 6 does not make it possible to sufficiently suppress diffusion of p-type conductivity impurity atoms (Zn atoms) contained in the p-type cladding layer 6 into the active layer 4. In the case where the p-type conductivity impurities contained in the cladding layer 6 are Be atoms, if a positive strain is applied to the p-type cladding layer 6, a large number of Be atoms are diffused into the active layer 4, which will invite an increase of driving current at high-power operation.
In the semiconductor laser device in which the diffusion of impurity atoms into the active layer is suppressed, laser light is prone to be absorbed in regions in proximity of end faces of a cavity, and therefore COD is liable to occur in regions of an active layer in proximity of the emission end faces. For that reason, a reduction of maximum optical output during high power operation is caused. Consequently, sufficient long-term reliability cannot be obtained.
In the semiconductor laser device having a conventional window structure, which is disclosed in Patent Document 2, diffusion of Zn atoms into the undoped active layer 24 is performed by the sealed tube diffusion method using, as an impurity diffusion source, ZnAs2 containing Zn atoms having a relatively large diffusion constant with respect to AlGaInP materials, so that the bandgap energy of the impurity diffusion regions (window regions) is larger than the bandgap energy corresponding to the laser oscillation wavelength. Thereby, absorption of laser light in the regions in proximity of the cavity is suppressed, and the occurrence of COD in the regions of the active layer in proximity of the emission end faces is prevented.
However, in the semiconductor laser device with the conventional window structure, for the bandgap energy of the active layer in proximity of the emission end faces to be larger than the bandgap energy corresponding to the laser oscillation wavelength, Zn atoms are diffused into the impurity diffusion regions (window regions) of the undoped active layer 24 in proximity of end faces of the laser cavity, as described above. At this time, disadvantageously, a large number of Zn atoms present in the p-type inner cladding layer 25 are diffused even into the excitation region (active region) of the undoped active layer 24, which invites the increase of driving current at high-power operation and deterioration of long-term reliability.